Double-felted press

ABSTRACT

A double-felted press for a papermaking machine or the like comprises a pair of press rolls arranged to form a nip and an endless press felt trained around a segment of each of the rolls, the press felts passing through the nip. One of the rolls is a suction roll, and the suction zone is located entirely on the downstream side of the roll, relative to the direction of rotation of the roll, from the nip, is contiguous or in proximate relation to the nip, and is within the segment of the roll over which the felt for that roll is trained. A guide roll for the felt associated with the other press roll (the nonsection-roll is normally positioned such that the felt is separated from the web being pressed before the web reaches the suction zone of the suction roll but is arranged to be moved to a position training that felt around the suction zone of the suction roll so that the two felts turn around the suction zone of the suction roll and ensure that the web runs with the suction-roll felt. Normally, the two felts are trained to run together over the suction zone of the suction roll only upon startup of the machine.

United States Patent Jan Peter Nilson [54] DOUBLE-FELTED PRESS 1 Claim,1 Drawing Fig.

[52] U.S. Cl 162/358 [51] Int. Cl. D2l13/00 [50] Field of Search 162/358[56] References Cited FOREIGN PATENTS 1,178,691 9/1964 Germany PrimaryExaminer-Howard R. Caine Attorney-Brumbaugh, Graves, Donohue & RaymondABSTRACT: A double-felted press for a papermaking machine or the likecomprises a pair of press rolls arranged to form a nip andan endlesspress felt trained around a segment of each of the rolls; the pressfelts passing through the nip. One of the rolls is a suction roll, andthe suction zone is located entirely on the downstream side of the roll,relative to the direction of rotation of the roll, from the nip, iscontiguous or in proximate relation to the nip, and is within thesegment of the roll over which the felt for that roll is trained. Aguide roll for the felt associated with the other press roll (thenonsection-roll is normally positioned such that the felt is separatedfrom the web being pressed before the web reaches the suction zone ofthe suction roll but is arranged to be moved to a position training thatfelt around the suction zone of the suction roll so that thetwo feltsturn around the suction zone of the suction roll and ensure that the webruns with the suctionroll felt. Normally, the two felts are trained torun together over the suction zone of the suction roll only upon startupof the machine.

PATENTED SEPZI I971 INVENTOFI'. JAN PETER NILSSON BY 7 Wfl M his ATTORNEYS.

DOUBLE-FELTED PRESS BACKGROUND OF THE INVENTION This invention relatesto a double-felted press of the type used in papermaking machines.

A roll-type press in which each of two press rolls forming a nip betweenthem runs within a felt loop is usually known as a double-felted press.Heretofore, double-felted presses have been used primarily inpapermaking machines that operate at relatively low speeds, generally atspeeds not greater than 200 meters per minute. A double-felted pressusually provides greater dewatering capacity than a single-felted pressinasmuch as, among other reasons, the felts on both sides of the webreceive liquid from the web, and the effective distance that the liquidmust flow across the thickness 'of the web to a felt is reduced. Becauseof the lesser distance (about half the thickness of the web), theresistance to liquid flow is correspondingly less, and therefore thereis less chance that the hydraulic pressure developed in the press nipwill exceed the shearing strength of the web and destroy the webstructure by phenomenon known as crushing. This permits a relativelyhigh press loading in a double-felted press.

In double-felted presses of the designs proposed and used heretofore,there is a tendency for liquids expressed from the web to return to itnear the end of the press nip and just after the web leaves the pressnip. To a relatively large extent, some of the advantage of thedouble-felted press is often lost through rewetting of the web in thisway.

Another disadvantage of presently known double-felted presses arisesfrom the tendency for the felts associated with the respective pressrolls to leave the nip with different moisture contents. Usually, thefelts travel away from the press nip along divergent paths, and the webis carried on the felt that is the wetter of the two because of thehigher surface tension forces of the wetter felt. Consequently, there isa substantial rewetting of the web, the rewetting tendency being higher,of course, with the wetter felt than it would be if the web followed thedrier felt. With a papermaking machine that operates at a relatively lowspeed, say below 200 meters per minute, the web can sometimes be made tofollow the drier felt by manually guiding it over to the drier felt uponstartup, and thereafter the web will follow the drier felt. Onhigh-speed papermaking machines, however, the web will not remain on thedrier felt, even if it may be initially guided onto the drier felt bysome means, but will return to the wetter felt in operation. It appearsthat the ability to operate a machine with the web following the drierfelt is restricted to speeds less than 200 meters per minute, though theprecise breakover point will vary somewhat depending upon the basisweight of the web and upon the dryness of the felts as they leave thepress nip.

One expedient for keeping the web on the drier felt at relatively higherspeeds is to use a suction roll as one of the two press rolls, thesuction roll having a suction zone extending through the nip andsomewhat downstream from the end of the nip, the suction roll being theroll associated with the drier felt. The suction in the suction rollholds the web on the drier felt as it leaves the nip, so that the websill thereafter run with the drier felt. Such an arrangement is,however, not entirely effective inasmuch as the benefit of having theweb run with the drier felt is somewhat offset by the rewetting of theweb by reason of water being sucked from the wetter felt by the actionof the suction roll before the wetter felt is separated from the web.

SUMMARY OF THE INVENTION There is provided, in accordance with theinvention, a novel and improved double-felted press that is arranged sothat the web will follow a desired one of the two felts, usually thedrier felt, with a minimum of rewetting from the other felt, i.e., thewetter felt. The arrangement offers the advantage of ensuring that theweb will follow the desired felt, even at relatively high machinespeeds.

More particularly, a double felted press, according to the invention,comprises a pair of press rolls arranged to form a press nip and anendless press felt associated with each of the rolls, the two pressfelts, of course, being looped around the respective rolls and passingthrough the press nip. The term press felt isused herein in the broadersense of any suitable endless, flexible, permeable web, includingfabrics made of various materials, such as-textile fibers, plasticfibers or filaments, or metallic wire. One of the two press rolls is asuction roll in which the suction zone is located, in accordance withthe invention, in a position entirely outside of the press nip but inproximate or contiguous relation to the downstream end of the press nipand within the portion of the suction roll over which the press feltassociated with it is trained.

By locating the suction zone entirely outsideof the press nip, thetendency for the suction to rewet the web by sucking liquid from thefelt associated with the other roll of the press roll pair issubstantially eliminated. On the other hand, the suction zone is locatedin a position where it is effective to hold the web on the feltassociated with the suction roll, which will usually be the felt which,in the particular machine, had the higher dryness of the two felts ofthe press.

Further, in accordance with the invention, provision is made in thepress for running both felts over the suction zone of the suction rollto ensure that the web, during the so-called tail-threading" uponstartup of the machine, is made to follow the felt associated with thesuction roll. In other words, the web is guided in a sandwich betweenthe two felts over the suction roll. In particular, the felt associatedwith the nonsuction roll of the roll pair is led from the nip to a rollthat can be adjusted from a normal position in which the felt divergesfrom the felt associated with the suction roll upon leaving the nip anda position in which that felt is trained with the suction roll felt forsome distance from the nip, such distance includ ing the suction zone ofthe suction roll. Thus, the operation of tail-threading is accomplishedwith the two felts running together over the suction zone with the webbetween them to ensure that the tail of the web follows the feltassociated with the suction roll at the outset. After the machine isbrought up to operating conditions, the adjustable roll is shifted backto the normal position so that the felt associated with the nonsuctionroll of the roll pair diverges from the nip and leaves the web supportedor carried on the suction roll felt as the suction roll felt traversesthe suction zone.

DESCRIPTION OF THE DRAWING For a better understanding of the invention,reference may be made to the following description of an exemplaryembodiment, taken in conjunction with the FIGURE of the accompanyingdrawing, which is a side view, in generally schematic form, of a portionof the press of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENT The double-felted press shown in thedrawing comprises two endless felts 1 and 3, each of which is arrangedto travel in an endless path over guide rolls 5. The drawing illustratesonly portions of the two felts, and those skilled in the art willreadily understand that the felts travel along an appropriate pathprovided by additional guide rolls that are not shown in the drawing.Each of thepress felts 1 and 3 will, of course, have cleaning and dryingapparatus of any suitable type, also as well known to those skilled inthe art, so that the felts return to the press nip in a relatively cleanand dry state and capable of adsorbing liquid pressed from the web inthe press nip.

The felts l and 3 pass together through a nip defined between a pair ofpress rolls 7 and 9, respectively, the roll 9 being a suction pressroll. The fibrous web, designated by the reference numeral 13, to bepressed is led through the nip 11 between the two felts 1 and 3, and issubjected to the squeezing action of a nip pressure exerted by urgingthe rolls 7 and 9 toward each other in a suitable manner, thereby toexpress liquid from the web 13. Although the web 13 is shown as beingdelivered in an open draw, that is, unsupported by any felt or

